The \( V_B^- \)  center in the hexagonal boron nitride is a defect formed by the loss of a boron atom at its lattice site and it is surrounded by three equivalent nitrogen atoms. So far, this defect is considered to be a spin triplet in both the electronic GS(\( {}^3A \) ) as well as the ES(\( {}^3E \) ) [1]}, [2]}, [3]}, [4]}. Other energy levels are still under debate, but it is now well established through experiments and theoretical calculations that at least one metastable state \( {}^3A \)  is located between the ground state and the excited state, and long lifetime of the metastable state leads to low quantum yield, \( \sim \)  0.03\( \% \)  [3]}, [4]}. Similar to the nitrogen vacancy color center (\( NV^- \) ) in diamond, due to the presence of non-radiative transitions and spin selective relaxation through an intersystem crossing to \( {}^1A \) , optical excitation-emission cycles induce the polarization of electron spin into the \( m_S = 0 \)  spin state [1]}, [8]}, [9]}. Furthermore, the spin selective nature of this process also creates a difference in the photoluminescence signal that depends on the spin state. In particular, the photoluminescence intensity is higher at \( m_S = 0 \)  than at \( m_S = \pm 1 \)  spin state [1]}.
